1. Field of the Invention
The present invention generally relates to an inkjet printer and, more particularly, the inkjet printer of a type utilizing a curtain of electric field for expelling droplets of inking material towards a recording medium.
2. Description of the Related Art
U.S. Pat. No. 4,717,926, issued Jan. 5, 1988, to Hideo Hotomi, one of the inventors of the present invention, and assigned to the same assignee of the present invention, discloses the inkjet printer utilizing a curtain of electric field (hereinafter referred to as an electric field curtain) for successively expelling droplets of fluidic inking material from a nozzle towards a recording medium. For the purpose of discussion of the principle of operation of the inkjet printer of the type disclosed therein, FIGS. 34 and 35 of the accompanying drawings illustrates, in schematic longitudinal sectional representation, a printer head used in such inkjet printer.
Referring to FIGS. 34 and 35, the printer head, generally identified by 200, comprises a ink reservoir 201 made of electrically insulating material and having a nozzle 202. The nozzle 202 has one end communicated with the bottom of the ink reservoir 201 and the opposite end formed with, or otherwise shaped to provide, an orifice 203. The ink reservoir 201 accommodates therein a quantity of fluidic inking material 204 which may be either a high resistance ink containing electrically charged coloring particles or an electrically chargeable ink. The nozzle 202 has first and second electrodes 205 and 207 disposed exteriorly therearound in the vicinity of the orifice 203 and spaced 180.degree. circumferentially with respect to the longitudinal axis of the nozzle 202. The first electrode 205 is electrically connected with an alternating current source while the second electrode 207 is grounded through a switch 208.
In the illustrated printer head disclosed in the above mentioned U.S. patent, so long as the switch 208 is open as shown in FIG. 24, a force F.sub.2 tending to expel the inking material 204 outwardly from the orifice 203 in the nozzle 202 and a force F.sub.3 of surface tension tending to drag the inking material 204 inwardly of the nozzle 202 against the force F.sub.2 act on portion of the inking material 204 retained within an exit area of the nozzle 202 adjacent the orifice 203 and encompassed by the first and second electrodes 205 and 207. If these forces F.sub.2 and F.sub.3 are in equilibrium with each other, that portion of the inking material 204 will not be expelled outwardly from the orifice 203.
However, upon closure of the switch 208 as a result of application thereto of an image signal supplied from a memory unit 209 through a control unit 210 as shown in FIG. 35, a force F.sub.1 induced by the electric field curtain (hereinafter referred to as an electric field curtain force) is developed in the electrically charged particles contained in the inking material 204 and, therefore, that portion of the inking material 204 within the exit area of the nozzle 202 adjacent the orifice 203 and encompassed by the first and second electrodes 205 and 207 is expelled outwardly from the orifice 203 towards a recording medium 211 in the form of an ink droplet 204a which is subsequently deposited on the recording medium 211. The electric field curtain force referred to above stands for a Coulomb force imposed on the electrically charged material by an alternating electric field which is unequal in time and space.
Thus, the inkjet printer disclosed in the above mentioned U.S. patent is so designed and so structured that, when an alternating voltage is applied to the first and second electrodes 205 and 207 in the printer head 200 in response to the image signal to form the electric field curtain, the inking material 204 can be expelled outwardly from the nozzle 202 in the form of ink droplets by the action of the electric field curtain so developed.
Accordingly, the inkjet printer utilizing the electric field curtain as described above is advantageous in that, as compared with well-known printers employing a stem system in which a piezoelectric element is utilized within the ink reservoir to apply to the inking material within the ink reservoir a pressure necessary to expel the inking material outwardly from the nozzle, a Caizer system and a pulse jet system, a relatively large change in volume can be available for a small work surface area and, therefore, the apparatus as a whole can be manufactured compact.
However, while a space 212 left within the nozzle 202 in the vicinity of the orifice 203 as a result of the ink droplet 204a having been expelled outwardly from the orifice 203 is required to be replenished by the remaining inking material 204 within the nozzle 202, the illustrated printer head 200 is not provided with a positive delivery means for positively delivering the remaining portion of the inking material 204 towards the orifice 203 to fill up the space 212 and, therefore, a relatively long time is required to fill up the space 212 with the remaining portion of the inking material 204. The time required to fill up the space 212 with the remaining portion of the inking material 204 may be more or less reduced if the switching frequency response is increased, however, the increase of the switching frequency response is limited.
The force necessary to expel the inking material 204 outwardly from the orifice 203 depends solely on the force induced by the electric field curtain developed between the first and second electrodes 205 and 207 and, therefore, a relatively high voltage has to be applied to the electrodes 205 and 207 in order for that portion of the inking material in a static state to be expelled outwardly from the orifice 203. This renders it difficult to employ a relatively low voltage for the inking material to be expelled outwardly, the employment of the relatively high voltage for this purpose resulting in the production of relatively large ink droplets which are attributable to the recording medium bearing relatively large dots with reduction in tone. Also, since an integrated circuit in a driver for each dot requires a relatively high voltage, rendering the inkjet printer as a whole to be costly.